Under Pressure

Doug Levin is the Associate Director for the Center for
Environment and Society at Washington College in Chestertown, Maryland,
and is an expert in underwater exploration technology, as well as
designing fun programs that teach complex engineering concepts.

This weekend I flew to Charleston, South Carolina. A short time after takeoff, the pilot came on the loudspeaker and proclaimed we had reached our cruising altitude of about 36,000 feet. I looked out the window down to the land below and realized that (coincidentally) I was looking at the equivalent of the distance to the deepest known point in Earth’s oceans, the Challenger Deep. That was mind blowing. Then, I began thinking about the difference of being 36,000 feet up in the air and that same depth underwater. If the flight attendant decided to open a window to let in some fresh air, there would be a frightening whooshing sound as the air in the plane, and everything not fastened down, was sucked into the lower pressure outside. In short order, we’d succumb to the lack of oxygen at this altitude if we didn’t quickly apply the yellow drop-down masks.

How different a scenario if we were in a submarine under the equivalent amount (36,000 feet) of water. At the Earth’s surface we have evolved to a level of comfort living under the pressure of air. Aside from the matter that we can’t breathe underwater, we also have a difficult time with increasing water depths due to increasing pressure related to its viscosity. Viscosity is the degree to which a liquid resists the tendency to flow–for example honey is more viscous than water when they are at the same temperature; it is more resistant to flow. Viscosity tends to increase at greater pressures. In easy terms, think of it this way: Which would you prefer…getting hit by a wall of air moving 60 mph or a wall of water moving at the same speed? I’m thinking a wall of air, because it wouldn’t necessarily knock me over but the wall of water would sweep me away.

Ok, so now as you drop into the depths of the ocean, for each 33 feet,
or 10 meters, you experience the equivalent of the pressure on your body
you feel at the Earth’s surface. At 66 ft (20m) you’d feel twice the
pressure; at 99ft (30m) three times, and so on. Now think about this:
At the Challenger Deep, the water depth is on the order of 36,000 feet
(12,000 m), or 36,000/33 = 1,090 atmospheres. So, if we were able to
step out of a sub at the deepest point in the ocean, we would feel over a
1,000 times more pressure than when we entered the sub at the ocean
surface. According to John Roach, who was writing for National
Geographic in 2005, this would be the equivalent of 50 airliners sitting
on top of you. Ouch.

Years ago, I was working for an oil company taking sediment samples in
water depths of about 1,000 ft (330m/30 atmospheres) using a piston
corer. For kicks we would write our names on a Styrofoam cup and send
it down with the sampler. When we retrieved the sample, we’d get our
cup back, miniaturized by the water pressure at depth. Our crushed cups
from thirty years ago are long gone, so I Googled “crushed Styrofoam
cups” and found this picture on the Internet. This is Chris Trahan’s
photo of a 16oz Styrofoam cup that they sent down to 9500ft (2900m)
inside of their Autonomous Underwater Vehicle while working on the
research vessel Northern Resolution in the Gulf of Mexico. The
Challenger Deep is more than three times deeper than what this cup
experienced. I hope that one of the sub pilots remembers to bring a
16oz Styrofoam cup with them for their dive. We might need a microscope
to see it when it comes back to the surface.

OK, I’ve got a question for you. There has been some talk that if we
sent an uncooked chicken egg to the abyss (deep ocean) it would come
back unbroken. What is your hypothesis (educated guess)? Do you think
the egg would break, or remain intact?

A 16oz Styrofoam cup, before and after being subjected to the pressures beneath nearly 10,000ft (2900m) of water.

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